1. Field of the Invention
The present invention relates generally to fabrication of electronic components, and more particularly, this invention relates to shaping of the junction between a hard biasing layer and the free layer of a magnetic read sensor of a hard disk drive.
2. Description of the Prior Art
In recent years there has been a constant drive to increase the performance of hard disk drives by increasing the areal data storage density of the magnetic hard disk. This is done by reducing the written data track width, such that more tracks per inch can be written on the disk. This naturally requires that the width of the read head be reduced so magnetic field interference from adjacent data tracks is not picked up. Read sensors, of which one type is referred to as a “spin valve”, developed to read trackwidths smaller than 130 nm depends upon the ability to ion mill the sensor to these very small dimensions, and to reliably lift-off the deposited layer materials.
One method used in the prior art for milling the read sensors is shown in FIG. 5 (Prior art). Preferably a single mill step at high incidence angle (0 to 15 degrees from normal incidence, i.e., perpendicular to the plane of the surface being milled) is used in conjunction with a 2-layer photoresist mask 40, having an upper layer 42 and a lower layer 44 to shape the read sensor 46. Commonly, the lower layer 44 of the 2-layer photoresist mask 40 is of a narrower dimension than upper layer 42 and of the trackwidth W 48 to form an undercut 49.
However, the limits of this technique are being reached because with trackwidths less than 130 nm, the width of the photoresist lower layer 44 becomes too small to support the upper layer 42. In response, techniques are being developed to use a photoresist mask which has no undercut. A single layer of photoresist or a multilayer structure can be used with no undercut formed.
After the read sensor or spin valve is completed, layers are deposited on both sides of the sensor. These generally include a seed layer, a stabilizing or hard bias layer and a layer of electrical leads. The junctions where these layers meet the layers of the spin valve sensor are very crucial to performance of the disk drive. For advanced spin valve read sensors used in magnetic recording heads, the hard bias to spin valve junction shape is especially critical. Different sensor designs and head designs call for various junction shapes. It is very desirable to vary the junction profile, and thus to affect the sharpness of the free layer edges, sharpness of the pinned layer edges, and the material on which the hard bias layers are grown, hence affecting device performance.
Referring now to FIG. 6, the stack of layers, referred to generally as a wafer stack 52, typically includes a first shield layer 54, a dielectric gap layer 56 and a first seed layer 58, upon which the remainder of the stack 60-68 is built. The milling or shaping process has typically involved cutting through the upper stack layers, completely through the first seed layer to reach the dielectric gap layer. It has been discovered that there are several disadvantages to this “complete milling” and many advantages to a “partial milling” operation in which some layers of sensor stack are left behind after the patterning operation, retaining a thin layer of material which covers the dielectric layer. In particular, when using complete milling, the thin dielectric layer underneath the sensor may become damaged by ion milling. This decreases manufacturing yields since there is more yield loss due to shorting between the sensor and the bottom shield. Secondly, the amount of material removed with complete milling is greater. There is thus more redeposited material that gets thrown against the sensor during milling processes. This is expected to give less clean junctions, with higher junction resistance. Also, the total milling time is naturally longer, and consequently, there is more chance of ESD damage.
Another disadvantage of complete milling is that at the end of the milling process, there are typically islands of patterned material left behind, rather than a continuous film of material on the wafer at all times, as there is with partial milling. Thus, there is more chance of charge buildup and potential ESD damage with complete milling.
Additionally, with partial milling, it is possible to stop at different points of the sensor stack (e.g. pinned layer), and achieve junctions of different shapes. Depending on the sensor film characteristics and hard bias/leads characteristics, this is expected to produce different sensor performance based on junction shape.
Also, it is an advantage that only a thin seed layer for the hard bias is required in the partial mill case. In contrast for the complete mill, a thick seed layer may be required in order to align the hard bias with the free layer. When depositing this thick layer, the amount of material deposited on the junction is significant. This can potentially increase junction resistance, and also leads to a larger spacing between the hard bias layer and the sensor, which is undesirable.
Thus, there is a need for shaped junctions and a method for achieving such junction shapes in spin valve sensors where the junction is achieved by partially milling through the sensor stack.